This invention relates to electric clamps such as toggle joint clamping devices for clamping work pieces, used in particular in the motor vehicle manufacturing or automotive field for holding panels or for clamping workpieces while they are being welded.
Usually, a toggle joint clamping device, as known and described for example in EP 0 778 107, comprises a clamping member, in the form of a clamping lever pivotally connected to a housing or support head, to rotate between a raised or open position, and a lowered or closed position in which the clamping lever lock or hold a workpiece again a shoulder surface or support frame. The clamping lever usually is connected, by means of an articulated linkage or toggle joint mechanism, to a linearly moving thrust member which at the same time is operatively connected to an actuator such as a pneumatic cylinder; the linear movement of the thrust member is thus transformed into a rotary movement of the clamping member, so that, as it moves from the open to the closed position with respect to a supporting structure, a work-piece can be firmly clamped during welding or similar working operations.
The pneumatically-operated clamping devices of the type previously described, however, present a number of drawbacks. In fact, in order to vary the working stroke of the pneumatic cylinder, it is necessary to substitute the same cylinder, or in any case mechanically modify the system, with consequent repercussions in terms of costs and loss of productivity for plants which make use of a relevant number of such clamping devices, due to a prolonged stoppage of the work cycle.
In addition, the pneumatic devices do not allow a sufficient control of the movement of the clamping lever in cases of emergency, such as to ensure the safety of operators and the same clamping device. Furthermore pneumatically operated clamping devices produce exhaust pressurized air, contamination and loud noise.
In order to obviate these drawbacks, as an alternative to pneumatic cylinders, for certain applications it has been proposed to use electric control means consisting of a single electric motor, as known and described for example in EP 0 255 853, EP 0 243 599, WO 99/50944, U.S. Pat. No. 6,354,580 and U.S. 2001/0013164.
In particular, EP 0 255 853 describes a toggle joint clamping device of the type mentioned above, in which an electric motor is axially aligned and directly connected to a nut screw device comprising a threaded spindle, to generate the linear movement of a thrust member necessary for the operation of the clamping device.
The remaining patents relate to clamping devices in which the nut screw device is connected to an electric motor by a nut screw or a screw shaft in which the same nut screw or the screw shaft is made to rotate by an electric motor through a gear unit for reducing and conforming the number of revolutions of the electric motor, to the required linear movement of the nut screw device.
By adopting an electric motor it is possible to change the pivotal angles of the clamping member, without calling for mechanical or manual interventions usually required for the pneumatically-operated clamping devices, by merely programming the software of an electronic control unit which controls the electric motor.
By electronically controlling the motor it is also possible to create a clamping system which complies with some safety standards in that, in the event of the clamping member encountering an obstacle, the latter may be detected making it possible to electrically generate a safeguard back movement for the opening of the clamping system, so as to prevent accidents, or injury or damage to objects or operators.
In addition, the use of electric motors offers the advantage in terms of economy and management, in that the efficiency of the electrical control is greater than that of the pneumatic control, due to the fewer energy transformations required.
However, the use of conventional electric motors, in particular the use of a conventional electric motor and a single gear unit for connection to the nut screw device, in which the electric motor is electrically and mechanically designed to provide low revolutions and an high driving torque of value sufficient to actuate the clamping device, and to lock a workpiece with a required clamping force, entails dimensional problems for the entire clamping device; in particular as far as the dimensions in width, in order to ensure a reliable operation of the motor and limit its overheating in the event of being used for a large number of operations, the electric motor must be oversized, compared to the required torque for operation of the clamping device. The device is consequently of such dimensions to jeopardize the possibility of having several clamping devices closely and side by side arranged in a limited space.
Moreover, in the event of malfunctioning of the single electric motor, it is necessary to stop the plant in which the clamping device is disposed, with consequent repercussions in terms of productivity and costs.
A further drawback occurs whenever, due to a possible malfunctioning of the electric motor, the clamping member remains locked in the closed position, in that the work-piece cannot be removed, or the clamping member cannot be safely opened whenever an accident occurs during the operative cycle.
To partially solve these problems in a clamping device of the previously mentioned type, EP 1 201 370 suggests the use of two electric motors connected by a simple gear reductor, to a nut screw device having a thrust member to operate the toggle lever mechanism of the clamping device.
Furthermore, to prevent unrequired forward rotation in the open position of the clamping member when power is disconnected to the electric motor, in particular for devices provided with long clamping arms, or in which the clamping arm is provided for moving heavy tools, braking systems or arrangements are required.
Although EP 1 201 370 suggests the use and a particular disposition of two electric motors to keep the housing for the electric motors of the same cross-wise dimensions of the housing for the toggle lever mechanism, nevertheless the use of conventional low-speed motors and a single gear mechanism prevents any possibility to substantially reduce the overall dimensions in respect to a conventional clamping device.
Lastly, a rigid pivotal connection between the toggle lever mechanism and the thrust member of the clamping device, due to the high inertia of the moving members, when the clamping lever suddenly stops against a work-piece, the entire device is subjected to shocks or impact forces which increase the wear on the moving members, reduce the life time and are negatively influencing any control at the stop of the clamping device.
The main object of this invention is to provide an electric clamping device suitably designed to solve the before mentioned drawbacks, by providing an electric clamping device having reduced overall dimensions, and in which the velocity and the inertia of the moving members may be gradually reduced in a controlled mode and in a very short space, at the stop.
A further object of the invention is to provide an electric clamping device of the above mentioned type which allows a greater stability, prevents an undue movement of the clamping arm in its open position, and at the same time provides safe working conditions with a simple and reliable solution.
Another object of the invention is to provide a clamping device provided with electric actuator means, in which the stop of the clamping arm and the clamping force may be positively controlled, to achieve the best safety and working conditions for the same clamping device.
Another object of this invention is to provide an electric clamping device provided with control means capable of exerting a braking action of the clamping member, when approaching the open and closed positions, so as to furtherly limit the effects of mechanical overstressing due to the high inertia of the moving members and/or of the fastened tools.
According to the invention an electric clamping device has been provided for clamping workpieces, comprising:
a housing for a toggle lever mechanism;
a clamping lever pivotally supported by the housing to rotate between an open and a closed position to lock a workpiece;
a toggle-lever mechanism inside said housing, said toggle-lever mechanism being operatively connected to said clamping lever and to an axially movable thrust member of a nut screw mechanism; and
electric motor means operatively connected to the nut screw mechanism,
wherein said electric motor means comprises:
at least one electric motor of the high rotational speed type; and
a geared mechanism operatively provided between said electric motor and said nut-screw mechanism;
said geared mechanism comprising a first gear reduction unit connected to the electric motor, and a torque adapter comprising a second gear unit between said first reduction gear unit and said nut screw mechanism.
Preferably, the nut screw mechanism is connected to the toggle-lever mechanism by elastically yielding means.
For the purpose of the present invention, xe2x80x9celectric motor of high speedxe2x80x9d refers to DC electric motors having a rotational speed equal to or higher than 5000 rpm, preferably comprised between 8000 and 15000 rpm, and a low output torque; said high speed motors have very small dimensions, while allowing a sufficient output power to actuate a workpiece clamping device.
Furthermore, xe2x80x9celastically yielding meansxe2x80x9d refer to any means suitable to provide a controlled elastic connection between the thrust member of the nut screw mechanism, and the toggle lever mechanism of the clamping device, such as cup shaped springs, helical spring, pad members in synthetic materials having a required modulus, or their combination.
According to a preferred embodiment, the electric motor means comprises first and second DC geared motors, of the type previously referred to, which are parallely arranged on opposite sides of the nut screw mechanism; each geared motor comprises a first gear unit connected to the nut screw mechanism by a respective second gear unit. Both electric motors are connected to an electronic control unit to be synchronously rotated at a same rotational speed, and to be short circuited to brake the same motors and gradually reduce their velocity, to decelerate the movements of the clamping device.
Preferably, the second gear unit comprises a first gear member having a first diameter, connected to the first gear unit, and a second gear member having a diameter equal to or smaller than the first gear member connected to the nut screw mechanism, and an intermediate pinion member between said first and second gear members; practically the second gear unit will act as a speed and toque adapter to conform the geared motors with the constructional features of the nut-screw mechanism and working requirements of the same clamping device; that is by merely changing the gears, it is possible to change the opening and closing velocity of the clamping arm, or the workpiece clamping force.
The nut screw mechanism may be of any desired type; it may be of self-locking type or reversible type which allows for a safety opening of the clamping arm when the electric motors are in a deactivated condition.